WOOD FOR THE TREES
129
THE VALUE OF INVERTEBRATES AS INDICATORS OF ANCIENT WOODLAND AND ESPECIALLY PASTURE WOODLAND KEITH ALEXANDER What is a wood? It may seem odd to ask such a question but the answer may be different in different circumstances. A basic broad definition might be: a concentration of trees, shrubs and other plants which have a character unique to that concentration - it is the concentration that makes the wood not the plants concerned. Special plants and animals may be associated which are not normally found where the same species of trees and shrubs occur in more isolated situations. This uniqueness of the flora and fauna is related to a number of features, but especially microclimate and - very importantly - management history. So basically we are talking about a concentration of trees. But there is a large spectrum of tree density possible, from dense closedcanopy through to very sparsely wooded. Certain woodland plants and animals may prefer particular densities of trees, particular light or shade levels. While the ecology of dense stands of trees, ie the conventional view of "woods", is the focus of much study, the sparser types of woodlands have been much neglected and are consequently poorly understood by most naturalists, ecologists, and conservationists - the dynamics of woodland communities in the more open situations is a particular problem area. Just as the term wood means different things to different organisms, so ancient woodland occurs in a wide variety of forms - viewed both from the perspective of the habitat requirements of the woodland organisms and from the management history point of view. Many species are associated with ancient woods because of their stability of damp shady habitat, while others are there for the long history of availability of timber in certain stages of decay. The shady habitat species are found in the denser ancient woodlands while the deadwood requiring species are found in ancient pasture-woodlands where the historic land-use wasn't primarily timber production, but pasture. Rackham (1996) has told us about the various traditions of tree management in Britain - emphasizing the importance of management history on the plant and animal communities. All sites effectively have unique management histories, although distinct patterns are apparent. If all are unique then, ideally, all should be conserved for all of their features. But this is impossible with all the pressures on the countryside. Conservationists need to assess the relative importance of different woodlands, to prioritise both protective measures and management plans. Ancient Woodland Indicators We know that where there has been a long history of woodland cover - ancient woodlands in the broader sense - species-richness is generally greatest. Those species or groups of species which are more or less unique to long-established woods have been used as "Ancient Woodland Indicators", and inter alia as indicators of habitat quality. The Indicator Species approach is an extremely valuable tool in site assessment. It identifies target species for survey, and thereby avoids the need for fĂźll Trans. Suffolk Nat. Soc. 32 (1996)
130
Suffolk Natural History, Vol. 32
species inventories - something that is impractical for invertebrates anyway. Where long species lists are available, this approach provides a means of interpreting them. It also focuses on the reasons why particular species are present in a particular site. Ideally the ecology of the species' selected as indicators should be relatively well-known, so that both survey and conservation management can be better targeted. Listings of indicator species which have been proposed are generally based on the experience of many naturalists over a long period of time. The scientific basis has not been tested and is probably not testable; but the depth of experience applied in producing these listings provides adequate validity. There are many pitfalls in using Indicator Lists. In particular the degree of association between a particular species and ancient sites may vary across that species' natural ränge. The value of vascular plants and lichens as ancient woodland indicators has been recognised for a long time, and lichens have proved especially important in increasing awareness of the importance of the pasture-woodland or wood-pasture type of woodland. Some invertebrates are also well-known to be good indicators of ancient woodlands, although well-known in a limited sense of within their own disciplines of entomology or conchology. It is only really in the last 20 years or so that invertebrates have become more widely accepted as good indicators of sites of nature conservation importance. Why are invertebrates especially valuable? There are many features of the biology and ecology of invertebrates that makes them particularly suitable for use as indicators of site quality for nature conservation. These have been reviewed by Kirby (1992): (i) Annual Life cycles: most invertebrates have annual life cycles, and therefore suitable conditions for breeding must be present each and every year. (ii) No long term resting stage, unlike many plants, with seeds which remain viable in soil for many years. (iii) Complex life cycles: different stages in the life cycle may have quite different habitat requirements, e.g. larvae develop in shady decaying wood while the adult favours sunny Clearings with flowers, where activity may be fueled from nectar and where potential mates may also be found. (iv) Highly specialised: some species feed at flowers of only a Single species of plant, while others breed only in red-rotten heartwood of oak and therefore require not only an oak tree but also the heart-rotting fungus Laetiporus sulphureus which causes the red-rot. (v) Microhabitats: invertebrates are all relatively small species; they may require features which seem to us very trivial, e.g. grass tussocks are the breeding site for certain species and an important overwintering site for others. (vi) Limited powers of dispersal: invertebrates exhibit a whole spectrum of mobility. Many are actually flightless, even certain beetles and other insects belonging to taxonomic groups normally regarded as possessing functional wings. Others are weak fliers, while some only fly under warm moist still air conditions.
Trans. Suffolk Nat. Soc. 32 (1996)
WOOD FOR THE TREES
131
(vii) Warmth loving: even woodland invertebrates often like warmth and sunshine, and may depend on it for the completion of the life cycles. (viii) Importance of structure: the physical features of a habitat affect the local climate - shelter, aspect, whether sunlight is reaching ground level, etc. (ix) Microclimate: scale - the piece of the world an invertebrate inhabits may be so small that the temperature, humidity and other conditions within it may be quite different to that experienced by larger animals like us. (x) Population size: the importance of being able to find mates and breed in sufficient numbers to maintain viable populations. Small populations may be particularly sensitive to loss through predation, adverse weather or changing management practices. It is mobility which actually limits certain species to ancient woodlands, as the other features listed above can be catered for in secondary woodlands. If the species is unable to get to the apparently suitable secondary woodland, then the new site is not colonised! There is actually very little about ancient woodlands that is special to them other than the species trapped within their bounds by their relatively poor ability to colonise new sites. Thus invertebrates can be very sensitive indicators of both present and past conditions in a particular site, and therefore can make very good indicators of the nature conservation value of a site. The invertebrate fauna says a great deal about a site; remember, beasts know best! Groups which have been analysed to produce lists of indicators tend to include the more glamorous species, large and showy ones, species which have attracted more attention from naturalists. The taxonomic basis reflects the focus of naturalists not nature. As I suggested at the very start, particular invertebrates may be indicators ot different things, e.g. a long and unbroken history of dense woodland cover, continuity of foodplants, of deadwood habitats. Where lists of indicators have been compiled there has been a divergence of approach. Some group specialists have favoured the idea of "primary woodland", ie woodland which has never been cleared of trees during the present interglacial and which therefore are direct links with the pre-Neolithic "wildwood". This has been the case with molluscs and hoverflies. Others have stuck with the idea of ancient woodland, as with the moth list. This is really a question of author's preference rather a real ecological distinction. Few authors have drawn up criteria for the selection of species which may be used as indicators of ancient woodlands. Speight (1989) provides a list of proposed criteria for drawing up a list of deadwood-breeding insects which might be used in identifying European forests of international importance to nature conservation. The species should be: 1. associated with the dominant, long-lived climax tree species of European forests; 2. dependent upon dead wood of senescent and dead trees for their habitats; 3. regarded as being today excessively localised in their European distribution; 4. of moderate to large size; 5. relatively easy to find; 6. relatively easy to determine.
Trans. Suffolk Nat. Soc. 32 (1996)
132
Suffolk Natural History, Vol. 32
The last three criteria are of course to do with the practical implementation of the list rather than its scientific value, while the first three relate to ecology and status. It is a useful set of criteria, but has its problems. Primary woodland molluscs Kerney and Stubbs (1980) provided a list of nine molluscs which are considered to be good indicators nationally of primary woodlands. Only five of these are known from Suffolk, although a sixth may well also be present in the county (see Fig 1). The habitat associations of most molluscs have been known for Species Point Snail Acicula fiisca
Where found permanently moist & well-sheltered situations in woodlands
Suffolk Status 5 sites, all in floodplains
Striated Whorl Snail Vertigo substriata
permanently moist & well-sheltered situations in woodlands
all fen or old hedgebank situations, in Breckland and coastal strip
English Chrysalis Snail Leiostyla anglica Plaited Snail Spermodea lamellata
Atlantic zone speciality centred on British Isles Atlantic zone speciality centred on British Isles
Mountain Bullin Ena montana
active amongst ground flora and ascends smoothbarked tree trunks, especially beech and ash
absent absent mostly in old hedges, but also floodplain situations, and one ancient wood Bulls Wood
Greater Pellucid Glass permanently moist & well- absent, although not Snail sheltered situations inconceivable Phenacolimax major in woodlands Ash-black Slug Limax cinereoniger
ascends tree trunks in warm moist weather to feed on fungi, sheltering at other times within decaying wood, under bark, etc.
discovered only recently in Suffolk at Wolves Wood, near Hadleigh
Slender Slug Limax tenellus
generally associated with large old trees in humid situations, feeding on fruiting bodies of wood decay fungi
not yet found in county, but potentially present
Brown Snail Atlantic zone speciality absent Zenobiella subrufescens centred on British Isles Figure 1: Slugs & Snails Indicative of Primary Woodland (from Kerney & Stubbs 1980).
Trans. Suffolk Nat. Soc. 32 (1996)
WOOD FOR THE TREES
133
many years, and Boycott's (1934) major paper brought together the accumulated knowledge at that time. Reading that paper, one appreciates how little our knowledge has advanced. The association of certain species with relatively undisturbed natural habitats was well known at that date, and it was already suggested that the occurrence of certain of the Limax spp provided evidence that a wood was on a "primeval" site - what we today refer to as primary woodland, with direct links to the Wildwood. These molluscs are associated with undisturbed moist situations. In Gloucestershire, where the ancient beechwoods are generally too dry, English chrysalis snail Leiostyla anglica and striated whorl snail Vertigo substriata are confined to wet flushes within these woods. In Ireland all habitats are sufficiently moist and English chrysalis snail can be found under rocks on the limestone pavement of the Burren! However, Suffolk is relatively dry for Britain and one might expect the ancient woodland molluscs to more closely associated with the damper conditions found in woodlands. Surprisingly some of these species occur in old hedgerows in the county: striated whorl snail Vertigo substriata and mountain bullin Ena montana, presumably indicating the origin of these hedges as remnant strips of otherwise cleared ancient woodlands. More predictably, when ash-black slug Limax cinereoniger was finally discovered in the county it was in a large ancient semi-natural woodland. The one missing species which is likely to occur in the county is slender slug Limax tenellus, a species like ash-black slug associated especially with ancient pasture-woodlands over much of its ränge. Perhaps it should be sought in those relict pasture woodlands which have been found to be relatively species-rich in other indicator groups? Deadwood Beetles Some 800 species of beetle are known to breed in association with decaying wood in the British Isles (Alexander, in prep.). About a quarter of these (193) have been suggested as being particularly associated with sites which have had a long and unbroken history of suitable deadwood habitats (Harding & Rose 1986). Suffolk is moderately rieh in these more restricted species and has at least 82, ie c.42% of them. It is likely that many more species are also confined to ancient woodlands, in parts of Britain at least. Examples of the more widespread species which breed in deadwood, but which are not true ancient woodland ones, include the distinetive wasp beetle Clytus arietis, lesser stag beetle Dorcus parallelepipedus and the red-headed cardinal beetle Pyrochroa serraticomis. At the other extreme, Suffolk has some of Britain's rarest deadwood beetles. The darkling beetle Prionychus melanarius is almost common in Staverton Park, but is known from only a handful of sites elsewhere in Britain - even Britain's liehest site, Windsor Great Park and Forest, doesn't have this species. Other Red Data Book species reported in the county include the stone beetle Euconnus pragensis - only known in Britain from Staverton and Windsor, and the false click beetle Aulonothroscus brevicollis at Icklingham Plains and only some 5 or 6 localities elsewhere.
Trans. Suffolk Nat. Soc. 32 (1996)
134
Suffolk Natural History, Vol. 32
An "Index of Ecological Continuity" has been devised for assessing the relative importance of sites for deadwood beetles in lowland Britain (Alexander 1988, Harding & Alexander 1994). The following Index scores are achieved in sites in Suffolk: Bradfield Woods Ickworth Park Shrubland Park Icklingham Plains Staverton Park
4 6 27 34 49
Predictably, it is the ancient pasture-woodland sites which are richest. After all, they have not been managed primarily for wood products for centuries! Interestingly, Ickworth and Shrubland Parks produce quite different Indices. They have both had checkered histories, the current parks having been enclosed from former agricultural land (O. Rackham, pers. comm.), but clearly the history of Shrubland has permitted better survival of deadwood beetles. Primary woodland hoverflies Certain hoverfly species have also long been known as being associated with relict old forest areas and ancient woodlands. Stubbs & Falk (1987) list 50 hoverfly species which are believed to be more or less confined to primary woodlands. The list is a composite one, inclduing not only those species which breed in decaying wood but also a variety of species which develop in live plant material or feed on aphids. This mix of ecological types, plus a general requirement of the larval stages for relatively moist conditions, leads to a different emphasis on sites. The conventional ancient woodlands, ie the dense ones, are often richer in primary woodland hoverflies than the pasture woodlands. Examples of species which are associated with good quality sites include the large black and red Brachypalpoides lenta, which breeds in rot-holes in trees, and the bee mimic Criorhina asilica, which is thought to breed in wet-rot of decaying tree roots. Higher quality sites will have species such as another bee mimic, Pocota personata, which has a very restricted British distribution and accordingly has Red Data Book status. It breeds in rot-holes high up in the trank or main boughs of overmature trees. Ancient Woodland Macro-Lepidoptera The larvae of moths and butterflies are mostly foliage feeders and so the distributions of these insects are very related to the presence or absence of plant species. However, some of these foodplants are actually ancient woodland species themselves, and so the insects are necessarily so too. Other species are associated with ancient woodlands despite the larval foodplants being more widespread. In the 1980s Peter Marren produced a listing of 75 species of butterfly and larger moth which appeared to be indicators of ancient woodland in central southern England. Old hedgerows were included within the definition as they can be derived directly from ancient woodland and are effectively linear woods. So far as I am aware, Marren's work has never been published.
Trans. Suffolk Nat. Soc. 32 (1996)
WOOD FOR THE TREES
135
The larvae of merveille du jour Dichonia aprilina feed on buds and leaves of oak in woods and pasture-woodlands, and so this moth is effectively an oakwood speciality, but it is a relatively mobile species and readily colonises new oak plantings. It is therefore relatively widespread and not particularly associated with ancient oakwoods although almost invariably present in them. In contrast, species such as the mocha Cyclophora annulata is dependent on mature field maple in woods and has only occasionally been found on hedgerow maples. Campanula pug Eupithecia denotata is confined to ancient woods through its foodplant Campanula trachelium, as is drab looper Minoa murinata the larvae of which feed on wood-spurge. The great oak beauty Boarmia roboraria is one of the classic moths of extensive ancient oakwoods. These species all feature on Marren's list. Other invertebrate groups Few other invertebrate groups have been examined systematically for associations with ancient woodlands, but certain individual species are acknowledged as being ancient woodland or old forest species. Amongst the false scorpions there is the rare Dendrochernes cyrneus which is mostly known from old forests and historic parks, and has been found in Ickworth Park. Other false scorpions live beneath loose bark on old trees and so are most often found in sites which are rieh in deadwood habitats. Amongst the bugs, Xylocoris cursitans lives beneath sappy bark on freshly dead timber and is mostly found in ancient woodlands. The Diptera include a wide ränge of species which are only known from ancient woodlands. Back to Basics Returning to the published lists of indicator species, there are a number of things to remember when trying to use them for site assessment. Firstly it is very important to understand what a particular list has been produced for, to avoid using it in a vaeuum of knowledge. It is essential to be aware of the key features of the habitat requirements of the suite of species in the list. It is especially important in using the deadwood beetle list to forget established concepts of what an ancient woodland or pasture-woodland should be - rieh sites may include places such as Icklingham Piain which isn't widely acknowledged as an ancient woodland but which clearly is an ancient pasture woodland. And, again with deadwood species, don't forget that deadwood is in reality usually decaying wood - the fungi are essential! Lists of deadwood species are especially valuable as they emphasize the distinetion between indicators of continuity of deadwood habitats from ancient woodlands, and therefore highlight a different ränge of sites of importance to nature conservation, sites which might otherwise be neglected by conservationists. Types of Situation with continuity of old timber By now it should be clear that conventional ancient woodlands are rarely of particular value for deadwood faunas, that it is the other types of ancient woodland that are the richest for these species. Trans. Suffolk Nat. Soc. 32 (1996)
136
Suffolk Natural History, Vol. 32
The following are the most important types of ancient woodland for deadwood communities and they are listed in Order of importance: 1. Old Forests & Chases e.g. Windsor, Sherwood, Epping 2. Historie Parks e.g. Moccas, Richmond & Staverton 3. Wooded Commons e.g. Burnham Beeches, Ashtead Common, Wytham Woods 4. Old Rough Pastures with Trees - winter fodder situations (i) fens such as Woodwalton and Chippenham, (ii) hill country situations such as the Cotswold scarp, eg Bredon Hill NNR, and the Lake District valley sides. (iii) The Breck e.g. Icklingham and Stanford PTA 5. River Floodplains, especially sequences of old willow and other pollards along the river bank and linking water course, but also linking hedgerow and field trees. 6. Ancient Woodlands e.g. Monks Wood NNR and Cirencester Park Woods. A final point Since different groups of species are associated with ancient woodlands for different reasons and therefore indicate different things about a site, it is clearly sensible when assessing the site's importance to use a ränge of indicator groups rather than rely on just one. All too often assessments of the relative importance of ancient woodlands have been based on just vascular plants, or, in the case of ancient pasture-woodlands, just on epiphytic lichens. Invertebrates provide an extremely valuable tool for such assessments and should be used alongside botanical assessments. References Alexander, K. N. A. (1988). The development of an index of ecological continuity for deadwood associated beetles. In Welch RC, Compiler. Insect indicators of ancient woodland, Antenna 12: 69-70. Alexander, K. N. A. (In prep.) An annotated checklist of the invertebrates of living and decaying timber in the British Isles. Boycott, A. E. (1934). The habitats of land mollusca in Britain. J. Ecol. 22: 1-138. Harding, P. T. & Alexander, K. N. A. (1994). The use of saproxylic invertebrates in the selection and evaluation of areas of relic forest in pasture-woodlands. British Journal of Entomology and Natural History, 7 (Suppl. 1): 21-26. Harding, P. T. & Rose, F. (1986). Pasture-woodlands in lowland Britain: a review of their importance for wildlife conservation. Institute of Terrestrial Ecology (NERC). Kerney, M. & Stubbs, A. (1980). The conservation of snails, slugs and freshwater mussels. Nature Conservancy Counil, Shrewsbury. Killeen, I. J. (1992). The land and freshwater molluscs of Suffolk. Suffolk Naturalists' Society, Ipswich.
Trans. Suffolk Nat. Soc. 32 (1996)
WOOD FOR THE TREES
137
Kirby, P. (1992). Habitat management for invertebrates: a practical handbook. RSPB, Sandy. Marren, P. Ancient Woodland Macro-lepidoptera in South England. Nature Conservancy Council (Unpublished). Rackham, O. (1996). This volume. Speight, M. C. D. (1989). Saproxylic invertebrates and their conservation. Nature & Environment Series No. 42. Strasbourg: Council of Europe. Stubbs, A. E. & Falk, S. (1987). Hoverflies as indicator species. Sorby Record, Special Series No. 6: 46-49. Keith Alexander, Estates Department, National Trust, 33 Sheep Street, Cirencester, Glos GL7 IRQ
Trans. Suffolk Nat. Soc. 32 (1996)